Electroplating baths and methods for electroplating gold alloys and a product thereof

ABSTRACT

Alkaline gold cyanide electrolyte solutions to be employed for depositing gold alloys by an electroplating process, such solutions having incorporated therein a mixture of an alkali metal silver cyanide and ethylenediaminotetraacetic acid and, optionally, alkali metal indium cyanide, processes of electroplating employing such solutions, and electroplated products produced thereby.

United States Patent Thomas J. Menzel Daytona Beach, Fla. 842,704

July 17, 1969 Sept. 21,1971

KDI Corporation Fairfax, Ohio Inventor Appl. No. Filed Patented Assignee ELECTROPLATING BATHS AND METHODS FOR ELECTROPLATING GOLD ALLOYS AND A PRODUCT THEREOF 6 Claims, No Drawings U.S. Cl 204/43 Int. Cl C23b 5/42 Field of Search 204/43, 44,

References Cited UNITED STATES PATENTS 11/1955 Spreter 204/43 Primary Examiner-G. L. Kaplan AnorneySughrue, Rothwell, Mion, Zinn & Macpeak ABSTRACT: Alkaline gold cyanide electrolyte solutions to be employed for depositing gold alloys by an electroplating process, such solutions having incorporated therein a mixture of an alkali metal silver cyanide and ethylenediaminotetraacetic acid and, optionally, alkali metal indium cyanide, processes of electroplating employing such solutions, and electroplated products produced thereby.

ELECTROPLATING BATHS AND METHODS FOR ELECTROPLATING GOLD ALLOYS AND A PRODUCT THEREOF BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the plating of gold alloys by the electroplating process and to electrolyte solutions to be employed in such a process.

2. Description of the Prior Art Heretofore, in the manufacture of gold-plated slip rings, and the like, to be employed in he production of electrical equipment, various methods of coating have been employed. For example, it has been the practice to provide basis coatings of copper and nickel followed by a gold coating having a thickness of 0.0003 inches. Alternatively, a basis coating of either copper or silver have been provided, followed by fold coatings of 0.003 to 0.040 inches. In such processes, it is necessary to machine the basis coatings prior to depositing the layer of gold. In one conventional method, wherein the rings are first copper coated and then machined, nickel plated and gold plated with a gold plating bath such as Sel-Rex- Autronex PC," the thickness of the gold coating is limited to a maximum of about 0.0003 to 0.0005 inches if satisfactory slip ring properties (i.e., the lack of brittleness and flaking caused by the buildup of internal stresses) are to be maintained. Since it is exceedingly difficult, if not impossible, to subject coatings of such thicknesses to machining operations, it is required, in the manufacture of slip rings, to employ coatings having thicknesses to about 0.003 to 0.040. However, such coatings as have been produced by plating baths previously known in the art have been marred by the formation of cracks, apparently due to the formation of an undesirable crystalline structure of the gold deposit. Further, it has proved difficult to produce coatings of the greater thicknesses required without introducing certain undesirable impurities into the gold deposits. Thicker coatings have been obtained through the use of special gold-plating baths containing chelating agents which allow deposits in thicknesses of up to about 0.040 inches. However, in such processes, a preliminary basis of copper or silver with intermediate machining is still required.

The use of potassium silver cyanide in potassium auro cyanide baths which may contain free potassium cyanide is known in the art. A patent to Rinker (RE 24,582) discloses that the presence of potassium silver cyanide results in the ability to produce brighter, harder, and thicker (up to 0.01 inches) gold deposits. However, the process of Rinker requires the use of plating baths containing high free potassium cyanide and silver contents. The presence of large amounts of free potassium cyanide results in an undesirable buildup of carbonates in the bath, which is further aggravated by increased temperature. Thus, the operating temperature of the Rinker process is limited to 80 F. Further, the amounts of silver employed in the Rinker baths, in contrast to the present invention, results in an undesirably impure gold deposit, and

\ the lower gold content of the Rinker baths limit the allowable current densities to be employed.

Further, a patent to Spreter et al. (2,724,687) discloses the addition, to an alkaline aurocyanide bath, of a metal to be alloyed with the gold in he form of its ethylenediaminotetraacetic acid salt. This bath, however, contains no free alkali cyanide and unlike the process of the present invention the ethylenediaminotetraacetic acid is added in the form of a metal salt. Spreter et al. teach that the advantage of this addition is that the bath does not age, as is the case with previously disclosed baths.

The addition of EDTA to alkaline aurocyanide plating baths is known to be ofimportance in the sequestering of polyvalent metal ions such as copper, tin and the like.

However, we have discovered empirically that the inclusion of both EDTA and certain amounts of dissolved silver in such baths results in the production of a gold deposit possessing-an isotropic crystal structure, and that such deposits are of great value in the manufacture and operation of gold slip rings.

SUMMARY OF THE INVENTION By the present invention, electroplating baths are provided which allow the deposition of thicknesses of gold of up to about 0.04 inches. At the same time, it is possible, employing the electroplating baths of the present invention, to eliminate the basis coatings required by electroplating baths of the prior art. Therefore, the present invention allows the production of suitably heavy gold deposits while at the same time greatly simplifying the plating process in that no machining of preliminary basis coatings is required.

This desirable result is achieved by incorporating into an alkaline aurocyanide bath a mixture of potassium silver cyanide and ethylenediaminetetraacetic acid (hereinafter EDTA).

DETAILED DESCRIPTION OF THE INVENTION The baths of the present invention are alkaline cyanide electroplating baths designed for the electrodeposition of an alloy comprising gold and silver. Electrolyte solutions comprising the electroplating baths of this invention comprise the following components in the proportions set forth below, all proportions being set forth in grams per liter:

which has been heretofore impossible to produce employing the baths of the prior art. It is this isotropic ring structure, produced from baths of the present invention, which make it possible to obtain crackfree, thick deposits and at the same time eliminate the added time and handling previously required by the necessity of machining of the basis coatings.

Further embodiments of the present invention are electroplating baths designed to produce alloys of gold, silver, and indium. The components of these baths and their proportions (in grams per liter) are set forth below:

Minimum Preferred Maximum Gold (as metal) 28.8 30.0 l30.0 Silver (as metal) 0.0l3 0.02 0.04 Indium (as metal) 0.013 0.02 2.6 Free Potassium 0.75 l5.0 30.0

Cyanide Potassium Carbonate l4!) 75.0 EDTA 7.0 [0.0 [5.0

The addition of indium to the baths of this invention as set forth above results in deposits possessing higher hardness and built-in" lubricating properties to the surface of coating. These properties are particularly advantageous, for example in the production of the slip rings mentioned above.

It is important to note that a particularly important function of EDTA in the above baths is the chelating or sequestering of polyvalent metal impurities. Hence, sufficient EDTA must be present in the bath to chelate such impurities as they are introduced.

Generally speaking, the plating bath of this invention may be operated at a current density of from 2 to 22 amperes per square foot of cathode surface, although it is preferable to operate at a current density of 10:5 amperes per square foot. The process of this invention operates at an efficiency of approximately percent-l00 percent.

The range of operating temperature for the baths of the present invention is about F. to about 130 F., preferably about 1 15 F. to about F. The pH of the bath is automatically maintained at about 12.2 by the potassium carbonate present.

It will be understood that the method employed in electroplating with the baths of this invention follows conventional practice in such matters as circulation of the electrolyte, replenishment of metal content and the like.

The baths of this invention are formed by dissolving the appropriate amounts of potassium aurocyanide, free potassium cyanide, and potassium carbonate in water. The silver is added during the course of the electroplating process as a mixture of potassium silver cyanide and EDTA. The deposition normally takes place over a period of about 20 hours, although a period of about 0.5 to about 50 hours is contemplated.

The following examples serve to illustrate the baths and method of operation of the present invention:

EXAMPLE I A slip ring is a mechanical device that is used to transmit electrical information from a stationary member to a rotating member or vice versa. Prior to the deposition of metal, the slip ring of this Example consists of cured epoxy resin and in machined to effect latitudinal grooves. Various copper leads extend from the core of the device and are separately exposed at the bottom of each groove. The bottom of the groove is coated with a conductive paint, which is removed from the end of the copper lead with a dental burr. The slip ring is then subjected to a preplate cleaning cycle e.g. pickling) and electrodeposited by immersion in an electrolyte having the following composition (in grams/liter) Gold (as metal) 33.0 Free potassium cyanide 30.0 Potassium carbonate 300 Silver (as metal) 0.02 EDTA 5.0

The gold and silver were added to the bath in the form of potassium metal salts. The conditions of treatment were as follows:

Temperature 120 F. Current density 12 apf Agitation vigorous Filtration constant Time of Electrolysis [8 hours Anodes Anode to cathode ratio Soluble gold 6 to l EXAMPLE II The procedure of Example I was followed except that the electrolyte solutions contained, in addition, indium in the amount of 0.02 grams/liter (added as potassium indium cyanide and EPTA was present in the amount of 10.0 grams/liter. A bright deposit was obtained in a thickness of 0.033 inches and possessed a hardness of 155 Knoop.

Deposits produced in accordance with the present invention meet the specifications of MIL-G-45204 B with respect to thickness, hardness and purity. This fact is extremely important, since the reliability of such products as slip rings depends, in great measure, upon the purity of the gold deposit coupled with good wearing qualities. The baths of the present invention produce gold deposits which possess this particular combination of properties and hence are particularly suited to be employed in the production of slip rings.

What is claimed is: 1. An aqueous alkaline cyanide gold-plating bath comprising from about 28.8 to g./l. of gold, added as potassium gold cyanide, about 0.0l3 to about 0.04 g./l. of silver, added as potassium silver cyanide, about 0.75 to about 30.0 g./l. of free potassium cyanide, about 14.9 to about 75 g/l of potassium carbonate, and about 2.0 to about 15.0 g./l. of ethylenediaminetetraacetic acid.

2. A process of producing a gold alloy coating which comprises electroplating the alloy from an aqueous alkaline cyanide gold-plating baths as set forth in claim 1.

3. A gold alloy-plated article produced in accordance with claim 2.

4. An aqueous alkaline gold-plating bath as set forth in claim 1 wherein the amount of ethylenediaminetetraacetic acid is present in an amount of from about 7.0 to l5.0 g./l. and which contains, in addition, about 0.013 to about 0.04 g./l. of indium, added as potassium indium cyanide.

5. A process of producing a gold alloy coating which comprises electroplating the alloy from an aqueous alkaline cyanide gold-plating bath as set forth in claim 4.

6. A gold alloy-plated article produced in accordance with claim 5. 

2. A process of producing a gold alloy coating which comprises electroplating the alloy from an aqueous alkaline cyanide gold-plating baths as set forth in claim
 1. 3. A gold alloy-plated article produced in accordance with claim
 2. 4. An aqueous alkaline gold-plating bath as set forth in claim 1 wherein the amount of ethylenediaminetetraacetic acid is present in an amount of from about 7.0 to 15.0 g./l. and which contains, in addition, about 0.013 to about 0.04 g./l. of indium, added as potassium indium cyanide.
 5. A process of producing a gold alloy coating which comprises electroplating the alloy from an aqueous alkaline cyanide gold-plating bath as set forth in claim
 4. 6. A gold alloy-plated article produced in accordance with claim
 5. 